Preparation of polyaniline–tin dioxide composites and their application in methanol electro-oxidation
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- Pang, H., Huang, C., Chen, J. et al. J Solid State Electrochem (2010) 14: 169. doi:10.1007/s10008-009-0892-4
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Polyaniline–tin dioxide (PANI-SnO2) composites were prepared by chemical polymerization method, and characterized by scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, and X-ray diffraction. Due to the good stability in diluted acidic solution, PANI-SnO2 composites were selected as the catalyst support and second catalyst for methanol electro-oxidation. The electrocatalytic properties of the PANI-SnO2 supported Pt catalyst (Pt/PANI-SnO2) for methanol oxidation have been investigated by cyclic voltammetry, chronoamperometry, and chronopotentiometry. Under the same loading mass of Pt, the Pt/PANI-SnO2 catalyst shows higher electrocatalytic activity towards methanol electro-oxidation than Pt/SnO2 catalyst.
KeywordsPANI-SnO2 compositesPt/PANI-SnO2 catalystElectrocatalytic activityMethanol electro-oxidationElectrochemical properties
In recent years, great attention has been paid to the electro-oxidation of small organic molecules mainly due to their potential application as fuel in the anode reaction of fuel cells. Therefore, regarding the electro-oxidation of methanol and the construction of direct methanol fuel cell (DMFC), intense research efforts have been made in the last two decades [1, 2]. Although DMFC is proposed to be a kind of promising power source due to its high energy-conversion efficiency, some obstacles, such as low methanol oxidation kinetics and methanol permeation across the proton exchange membrane, still exist for its commercialization. It is well known that platinum is considered as the best single metal catalyst for methanol oxidation, but the poisoning of platinum catalyst by intermediates such as COads is the main reason for the low kinetics of methanol electro-oxidation [3, 4]. Nowadays, it was reported that the introduction of oxides (such as WO3 [5, 6], CeO2 [7, 8], RuO2 ) is efficient to improve the catalytic activity of Pt for the oxidation of methanol.
SnO2, one of the most widely used metal oxide catalysts for CO oxidation [10–12], is quite stable in diluted acidic solution. Because of its unique physicochemical properties, SnO2 has also been used as the catalyst or catalyst support for oxidation of various kinds of hydrocarbons . Preliminary studies indicated that the presence of tin oxides in Pt catalyst led to higher current densities in acid solution for the electro-oxidation of methanol in comparison with the case of pure platinum catalyst . They deduced that SnO2 in the vicinity of Pt nanoparticles could offer oxygen species conveniently to remove the CO-like species and free Pt active sites. However, for a good electrocatalyst, except CO-tolerance ability, electronic conductivity is also an important aspect, especially in real fuel cells. Therefore, as a semi-conductive oxide, the electronic conductivity of SnO2 particles still needs to be improved if it is used as the catalyst in DMFC.
It is well documented that the addition of conductive additives can enhance the electronic conduction between active materials during redox reactions [15–17]. The electronic conductivity of the conducting polymers, which is as high as the metallic conductivity, attracts great interest from an electrochemical viewpoint. Some conducting polymers have been studied for their catalytic behavior towards electrochemical reactions [18, 19]. Among this type of polymers, polyaniline (PANI) is one of the most studied conducting polymers because of its good electrical conductivity, environmental stability, and relative easy synthesis . Besides, it was reported that PANI could be a good conducting matrix for the dispersion of Pt catalyst particles in methanol electro-oxidation [19, 21]. However, to the best of our knowledge, there are no works focusing on the application of PANI-SnO2 composites in fuel cells, although the PANI-SnO2 composites have already been studied as the optical, gas sensing, and supercapacitive materials [22–24]. Therefore, the PANI-SnO2 composites were prepared in this paper and used as the catalyst support and the second catalyst for methanol oxidation. The electrocatalytic properties of the PANI-SnO2 composites supported Pt catalyst (Pt/PANI-SnO2) for methanol elelctro-oxidation have been investigated by cyclic voltammetry (CV), chronoamperometry (CA), and chronopotentiometry (CP).
Preparation and characterization of the PANI-SnO2 composites
SnO2 nanoparticles were synthesized by sol–gel method according to the same procedure mentioned in our previous work . Two grams of SnCl2·2H2O was dissolved in 120 mL ethanol and a suitable amount of 0.5 M Na2CO3 aqueous solution was added drop-wise under ultrasonic stirring to obtain a sol, then aged overnight. After filtration and washed with double-distilled water for several times, the resulted deposit was dried at 80 °C for 4 h, then grounded and calcinated at 450 °C in air for 3 h to obtain SnO2 nanoparticles.
Aniline monomer were added into 1 M HCl solution and ultrasonically treated for 30 min, then a specific amount of SnO2 powder (the molar ratio of aniline to SnO2 was 1:3) was added. The mixture was stirred and ultrasonically treated for 1 h each, then a definite amount of ammonium peroxydisulfate (APS) dissolved in 1 M HCl solution (the molar ratio of aniline to APS was 1:1) was added drop-wise within 1 h and the reaction time was 5 h under vigorous stirring. The resulted green dispersion was centrifuged and washed by 0.2 M HCl solution and double-distilled water for several times. Then washed by acetone and the resulted deposit was dried at 60 °C in vacuum for 4 h to obtain the PANI-SnO2 composites.
The surface morphology of the PANI-SnO2 composites was analyzed by scanning electron microscopy (SEM, JSM 5600 LV, 30 kV) and transmission electron microscopy (TEM, JEM-3010). The crystal structure was examined by X-ray diffraction (XRD, D/MAX-RA). Fourier transform infrared (FTIR) spectrum was recorded on a NICOLET 6700 spectrophotometer using KBr pellets.
Preparation, characterization, and electrochemical measurements of the Pt/PANI-SnO2 catalyst
PANI-SnO2 powder, 2.0 mg, was dispersed in 4 mL of double-distilled water and then mixed with 90 μL of 38.6 mM H2PtCl6 aqueous solution under ultrasonic stirring. The excess fresh NaBH4 solution was added drop-wise into the mixture and the solution color was changed from yellow to black. This suggested that the Pt/PANI-SnO2 hybrid catalyst was formed. A definite volume of the catalyst ink was then transferred onto the surface of the glassy carbon (GC) electrode by a micro-syringe. After drying in air, the electrode was coated with 5 μL of 0.05 wt.% Nafion ethanol solution.
For comparison, the Pt/SnO2 catalyst was also prepared and transferred onto the surface of the GC electrode according to the same procedure mentioned above.
The morphology of the Pt/PANI-SnO2 catalyst was characterized by transmission electron microscopy (TEM, JEM-3010).
The electrochemical properties of the Pt/PANI-SnO2/GC and Pt/SnO2/GC electrodes were investigated in 0.5 M H2SO4 + 1.0 M CH3OH aqueous solution by electrochemical methods, those were carried out on a CHI 660A electrochemical working station (Chenhua Instrument Company of Shanghai, China) at 25 °C. A standard three-electrode cell was employed with platinum wire as the counter electrode and saturated calomel electrode as the reference electrode. As the working electrode, the GC electrodes with an exposure area of 0.2 cm2 were used to support catalysts.
Results and discussion
Characterization of the PANI-SnO2 composites and the Pt/PANI-SnO2 catalyst
Electrochemical properties of the Pt/PANI-SnO2 catalyst
The PANI-SnO2 composites, which were used to support Pt particles for methanol electro-oxidation, were prepared by chemical polymerization method in this paper. The electrochemical properties of the Pt/PANI-SnO2/GC electrode have been investigated by cyclic voltammetry, chronoamperometry, and chronopotentiometry in 0.5 M H2SO4 and 1.0 M CH3OH aqueous solution. Comparing with the Pt/SnO2/GC electrode, the Pt/PANI-SnO2/GC electrode shows better electrochemical performance (larger ESA value, higher electrocatalytic activity and better anti-poisoning ability) under the same experimental conditions. These suggest that the Pt/PANI-SnO2 catalysts will come into being a promising candidate for methanol oxidation in DMFC.
This work is supported by Program for New Century Excellent Talents in University (NCET-04-0765), National Natural Science Foundation of China (50172014, 20675027), and Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (2001-498).